defense of his huge threatened latifundium and its people, began retreating at the end of May from left-bank Ukraine. Fighting the Cossacks with some measure of success he matched the ruthlessness of the rebels, but brought no resolution to the conflict and prevented a possibility of the considered negotiated solution. Wiśniowiecki commanded his army at the Battle of Starokostiantyniv in late July against Cossack forces led by Maksym Kryvonis; its indecisive outcome allowed Wiśniowiecki and his units to continue their westward movement. The death of Władysław IV on May 20 and the discordant interregnum had made the matters worse. Chancellor Jerzy Ossoliński and Voivode Adam Kisiel, a magnate influential with the Ukrainian population, advocated making the concessions necessary to placate the Cossacks. Other magnates, especially Wiśniowiecki, Aleksander Koniecpolski and Janusz Radziwiłł, demanded taking whatever radical punitive steps were required to quench the disorder. The convocation sejm was likewise indecisive and the newly appointed military chiefs incompetent, which resulted in a disastrous defeat at the Battle of Pyliavtsi in September. This allowed Khmelnytsky to move toward Lwów and Zamość, while the uprising expanded into Volhynia and Belarus. The situation made it possible for the self-styled Cossack Hetman Khmelnytsky to influence the royal election, during which John Casimir Vasa was chosen with his support. John Casimir and the peace nobility faction made offers and engaged in fruitless talks with Khmelnytsky, who had withdrawn into Ukraine. In February 1649 in Pereiaslav the self-confident Hetman, now speaking of total liberation of the "Rus' nation", negotiated with Adam Kisiel, but only a temporary suspension of hostilities was agreed to. Both sides pursued armaments and in spring military activities were resumed. The Polish units protecting Volhynia under Wiśniowiecki were able to defend their fortified encampment at Zbarazh in July and August against the overwhelming Cossack and Tatar armies. Their situation was getting increasingly desperate when the main Crown forces under King John Casimir approached in a rescue attempt. The King was surprised by the enemy at the Battle of Zboriv, but Ossoliński saved the Polish army by successfully negotiating with İslâm III Giray, the Tatar Khan, who was becoming worried about the Cossacks' growing power. Since the Lithuanian forces under Janusz Radziwiłł also entered the Dnieper region, Khmelnytsky consented to ceasing of hostilities on favorable for him, the Cossacks and the Eastern Orthodox Church conditions negotiated in the Treaty of Zboriv (the Cossack Hetmanate was officially